Diseases and Management of Crops under Protected Cultivation

(Diseases and Management of Crops under Protected Cultivation)
cations than anions, H + ions are excreted by the roots and rhizosphere pH decreases. Fluctuations
of pH of the ground around the roots of the order of 1.5 units of pH due to ammonium or nitric
nutrition have been reported in the literature (Barber, 1984). According to Ganmore-Neumann and
Kafkafi (1980, 1983), NH 4 is an undesirable source of nitrogen for tomato and strawberries when
the temperature in the root zone is greater than 30°C, due to its adverse effect on root growth and
plant development. The pattern of cationic uptake due to ammonium nutrition decreases the
uptake of other cations like Ca 2+ , Mg 2+ and K + .
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When NO 3 anions are absorbed, the plant takes up more anions than cations and the
excess of anions is palliated by a greater synthesis of carboxylates. During the carboxylation
process dicarboxylic acids (citric, malic, etc.) and OH - are produced.
Both the carboxylates and the hydroxyls can be exuded by the roots to the soil The exuded
OH - increase the pH of the rhizosphere The organic acids exuded by the roots increase the
availability of phosphorus since the carboxylates are specifically adsorbed to iron oxides and clays
of the ground, releasing therefore adsorbed phosphorus to the soil solution. The carboxylates can
also increase to the availability of iron and phosphorus by chelation: for example, citrate forms a
chelate with calcium, thus releasing phosphorus that is under the calcium phosphate form (Imas et
al., 1997).
According to this, NO 3 nutrition is recommended due to the greater organic acid synthesis
and enhanced cations uptake, whereas the ammonium nutrition is detrimental However, nutrition
with 100% nitrates would increase rhizospheric pH up to undesirable values - values of more than
8 have been registered - and this would decrease the availability of P and micronutrients by
precipitation. Therefore it is recommended to use a nitrogen mixture with 80% of nitrates and 20%
of ammonium to regulate pH.
5. The Fertilization Program
‣ Crop Nutrients requirements: It depend on crop specific needs, yields, methods of
growing (open / protected cultivation) and variety. The crop specific need may be asses by
mineral analysis of harvested part and vegetative biomass, ratio between N-P-K-Ca-Mg
and percent dry matter. Based on mineral analysis and the yield / plant ratio, one may
estimate the crop need for a specific yield and for each ton produced not proportional.
‣ Soil analysis: How much N, P, K, Ca, Mg to apply.
‣ How much nutrients to apply :
o Vegetables: Fertilization=soil deficit correction + crop nutrients requirement
(removed+plant)
o Field crops : Fertilization=soil deficit correction + removed by yield (harvested
o Orchards: Fertilization=soil deficit correction + removed by yield
No soil anylysis: The following factor may be used
N total x (1.2 -1.3) – depend on soil type i.e. lower for heavy soil and higher for
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